Receptors on rabbit neutrophils which mediate chemotaxis have been demonstrated by direct binding of a small peptide. Bacterial invaders make similar peptides and white cells move to the site of inflammation via interactions with this receptor in the first step of the immune reaction. BIBLIOGRAPHIC REFERENCES: Aswanikumar, S., Corcoran, B., Schiffman, E., Day, A.R., Freer, R.J., Showell, H.J., Becker, E.L. and Pert, C.B.: Demonstration of a receptor on rabbit neutrophils for chemotactic peptides. Biochem. Biophys. Res. Comm. 74: 810-817, 1977. Day, A.R., Radding, J. Freer, R.J., Becker, E.L., Aswanikumar, S., Corcoran, B., Schiffman, E. and Pert, C. B.: The synthesis and chemotactic activity of an intrinsically labeled, formylated tripeptide. FEBS Letters, in press, 1977.